The present invention relates to a transducer for a SAW arrangement wherein to provide unidirectional operation of the transducer, the transducer is formed of first and second interleaved sub-transducers respectively connecting to first and second phase voltages.
SAW (Surface Acoustic Waves) arrangements, particularly SAW filters, are electrical components which are based on the propagation of mechanical or acoustic waves in the surface of a piezo-electric substrate lamina. The conversion of electrical signals into surface acoustic waves and vice versa usually occurs by means of "interdigital transducers". In the simplest case, such transducers are composed of a series of thin metal strips vapor-deposited onto the substrate surface, these being referred to as finger electrodes and being interdigitally or alternatly connected to two busbars. When an alternating electrical voltage is applied to the busbar, a surface deformation of the substrate arises due to the piezo-electric effect, this deformation propagating as a mechanical wave in two opposite directions at right angles to the longest expanse of the finger electrodes. An electrical signal is again acquired from the mechanical surface wave by means of a further interdigital transducer. The transfer function of a SAW arrangement constructed in such a way can be largely prescribed by the geometry of the two transducers (i.e., by the plurality of finger electrodes, their lengths, widths, spacings, etc.). An optimally pronounced bandpass behavior is frequently strived for.
SAW arrangements having simple transducer types (referred to as bidirectional transducers) exhibit considerable losses, partially because the transmission transducer emits the surface waves in two opposite directions and because the reception transducer can output a maximum of one half of the incident energy to an electrical load. They also exhibit losses because the transducers dare not be optimally matched to the source or load since disturbing reflections (referred to as triple transit echoes) would otherwise occur within the transducer.
The described filter losses can be largely reduced when directional transducers are employed, these emitting the acoustic waves in the pass band in practically only one direction. Multiphase and group directional transducers (group type unidirectional transducer) have achieved practical significance. Multi-phase transducers are constructed in multilayer fashion and are therefore significantly more involved in terms of manufacture than a group type unidirectional transducer constructed in single-layer fashion.
As FIG. 2 shows, the finger electrodes in group type unidirectional transducers are arranged in a number of groups Ga, Gb, Gc, . . . . Each individual group Ga, Gb . . . is formed of two half-groups Ga1, Ga2; Gb1, Gb2; . . . which are usually identical and which, as is known, are shifted by n.multidot..lambda..+-..lambda./4 relative to one another. Therein, n is a whole number and lambda is the wavelength at the synchronous frequency. This transducer has two busbars Ph1 and Ph2. A further, and in fact common, meandershaped busbar or ground wire M is provided, the path thereof in the transducer being such that this further busbar separates half-groups lying proximate to one another from one another, as shown in FIG. 2. Dependent upon affiliation to the left (Ga1, Gb1, . . . ) or to the right (Ga2, Gb2, . . . ) half-group, the "hot" finger electrodes of these half-groups are connected to one of the two busbars Ph1 or Ph2, respectively. Accordingly, the one phase of the electrical signal to be fed in is applied to the one busbar Ph1. The other phase of this signal is applied to the other busbar Ph2, whereby these two phases have a phase difference of 90.degree. relative to one another at the synchronous frequency. A third phase, or a reference potential such as ground, is applied to the further busbar. Accordingly, the transducer can be thought of as being formed of two mutually interleaved sub-transducers, whereby the one sub-transducer comprises the half-groups Ga1, Gb1, Gc1, . . . and the other subtransducer comprises the half-groups Ga2, Gb2, Gc2, . . . .
The directional effect of this directional transducer then arises since the waves of the two sub-transducers in the forward direction add up, whereas they cancel in the other direction. In the forward direction, the topical and chronological phase differences compensate; in the reverse direction, they add up to 180.degree.. Such a directional transducer can be employed both as a transmission transducer as well as a reception transducer, and works nearly loss-free in both applications.
The above-described, known group type unidirectional transducer (FIG. 2) has two deficiencies associated therewith:
1. The ohmic resistance of the further busbar or ground wire M causes losses and a distortion of the frequency characteristic; and
2. Pronounced secondary lobes lie in or puncture the stop band.